Computer telephony, that is, the delivery of telephone calls over computer networks, has recently become a focus of attention due to the potential cost savings of sharing these modem high-bandwidth facilities for multiple uses. Because computer networks packetize signals and then mix such packetized signals (or more simply, packets) from many sources over a single link, networks can make more efficient use of communications resources than conventional circuits witched telephone systems. Furthermore, computer networks leverage the mass-production cost savings and technological advances of commodity products. This sharing of computer communications for non-computer signals therefore has the potential to greatly lower the cost of communications when used with telephone signals.
Computer network traffic from telephone, video, and other time-sensitive sources are generally referred to as real-time traffic because such traffic must arrive at a destination within a specified deadline. Real-time traffic generated from audio or video sources is usually generated in equally spaced time intervals. This type of periodic real-time traffic is referred to as isochronous traffic.
When isochronous traffic is digitized and combined with the sophisticated computer-processing compression techniques, the result is a significant reduction in bandwidth requirements. This use of computer technology to send telephone and video signals thereby results in even further cost savings.
However, conventional computer networks are not designed to handle real-time traffic. Collisions and congestion can induce delays and retransmissions, and can cause real-time traffic, such as video, audio, telemetry, and control signals, to arrive late at a destination, thereby missing a deadline. Furthermore, such collision-induced delays are stochastic by nature and therefore unpredictable. Isochronous traffic sources become bursty after traveling through such networks. As a result, the quality of telephone calls placed over the Internet and computer networks in general is very poor at present.
Ethernet computer networks, in particular, use a form of media access control known as Carrier Sense Multiple Access with Collision Detect (CSMA/CD), also sometimes known as Aloha. This protocol is described in detail by the IEEE Standard 802.3. It provides a very simple and effective mechanism for allowing multiple packet sources to share a single broadcast computer network medium. To transmit a new packet, a transmitter need only listen to the network to sense that no packet is currently being transmitted. As a transmitted packet is broadcast to all receivers on the local network, listening to the network for activity is trivial. If a transmitter wishing to send a packet senses that a packet is currently being transmitted, then the transmitter defers transmission until it senses that the network is inactive. Collisions naturally arise as part of this mechanism. The most common scenario leading to a collision is where two or more stations, which are deferring their own respective transmissions during the transmission of another packet, sense a lack of activity at nearly the same time. The protocol detects collisions, and then aborts and reschedules transmission of all packets for a random time later. This protocol, while simple and effective for computer traffic, introduces collisions and delays as part of its natural operation. In fact, overloading such a network causes the entire network to become unusable, resulting in a significant reduction in throughput.
Ethernet is now ubiquitous throughout the Internet within local-area computer networks, or intranets. The use of variable packet sizes. and Carrier Sense Multiple Access with Collision Detect for link access and control creates an even less predictable and less controllable environment for guaranteeing quality of service. This is of particular concern for wide-area real time traffic that must traverse a plurality of Ethernet networks in order to reach a final destination.